JAMES R. STEIDTMANN Department of Geology, University of Wyoming, Laramie, Wyoming 82070

Origin of the Pass Peak Formation and Equivalent Early Strata, Central Western Wyoming

ABSTRACT plex, and evidence from previous investigations is inconclusive with regard to timing. As a The Pass Peak Formation is a Tertiary result, reconstructions of the Tertiary geologic basin-flank deposit exposed throughout much history of this area are contradictory. A thor- of the Hoback Basin; a depression physio- ough investigation of the nature and distribu- graphically distinct from, but structurally con- tion of orogenic sediments and the relations of tinuous with, the northern Green River Basin. the various sequences to one another is neces- Two lithofacies of the Pass Peak Formation are sary if the Tertiary history is to be recon- identified. A northern quartzite conglomerate- structed. sandstone facies, possibly as thick as 3200 ft, The early Tertiary Pass Peak Formation is intertongues southward with a sandstone-silt- orogenic in origin, having been deposited in stone facies 1500 ft thick. This in turn extends response to local or regional uplift, or both. southward and intertongues with the Wasatch Determination of the source and distribution Formation. Arkosic sandstone and conglomer- of the Pass Peak sediments and an understand- ate intertongues with the Pass Peak on the east. ing of the sedimentary processes involved in Trends in grain size and sandstone composi- their deposition are fundamental to the inter- tion, and directional structures indicate that: pretation of the sequence of early Tertiary (1) a sedimentary source to the north supplied events in central-western Wyoming. The inves- quartzite cobbles and associated garnet-bearing tigation of the Pass Peak was undertaken with sand and (2) an igneous or metamorphic source, the following objectives: (1) to describe the or both, to the northeast supplied abundant formation and to map its distribution; (2) to arkosic debris. Regional geology suggests that determine and describe the provenance and the Pinyon Conglomerate to the north was the dispersal pattern of the sediments; and (3) source for Pass Peak sediments and the Pre- to determine the environment or environ- core of the Wind River Range to the ments of deposition. northeast was the source for the arkosic sedi- ments to the east. Previous Investigations Uplift in the Mt. Leidy Highlands, the Gros The Pass Peak Formation was named by Ventre Mountains, and the Wind River Range Eardley et al. (1944) from Pass Peak, a high during the early Eocene resulted in the deposi- point on the Hoback-Green River divide, tion of fanglomerate in the northern part of the where the rocks are exposed in a slide scar 1800 basin and coarse arkosic alluvial plain sedi- ft high (Fig. 1). The lowest yellow sandstone ments in the eastern part. The finer debris from was defined as the base of the formation and the north was carried southward onto a flood- was mapped over much of the southern and plain where it mixed with the finer sediments eastern part of the Hoback Basin and north- from the east. ward between the Gros Ventre and Hoback Ranges. Dorr (1958, Fig. 1) remapped the INTRODUCTION Pass Peak using the lowest conglomerate as the base. He placed the sandstones beneath this in Objectives the Hoback Formation or in his so-called The sequence of Laramide sedimentational "transition zone." The distribution of the Pass events in central-western Wyoming is com- Peak Formation, therefore, was limited to a

Geological Society of America Bulletin, v. 82, p. 156-176, 11 figs., January 1971 159

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V / J Poleozpe and Mesozoic \/s\ sedimentary rocks

Precombnan rocks

EXPLANATION

HOBACK Pass Bon d u r a n t

Big Pmey . I Utah Colorado

Figure 1. Major physiographic and geologic features of central western Wyoming. Area of investigation enclosed by dashed line. Adapted from Love et al. (1955).

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smaller area in the southern and eastern part was broken, and its lithology determined. Ap- of the basin. proximately, 50 were examined at each location. Keefer (1964) demonstrated that the Pass The "largest size" method, suggested by Petti- Peak Formation was involved in the faulting john (1957, p. 249), was used to obtain a clast- along the southernmost margin of the Gros size index. The maximum axes of the ten largest Ventre Mountains. Antweiler and Love (1967, clasts found at each outcrop were measured p. 7) described the occurrence of gold in the with large calipers. The ten measurements were Pass Peak Formation. On the basis of verte- averaged, and this result was used as the clast- brate evidence, Dorr (1969) determined that size parameter for that location. the age of the Pass Peak ranges from late Heavy mineral concentrates were obtained Graybullian (late early ) through by a centrifugation process described by Steidt- Lysitean (middle Wasatchian; that is, middle mann (1969b). The "line method" described early Eocene), and possibly into early Lost- by Ramesam (1966, p. 630) was used for making cabinian (early late Wasatchian, that is, late grain counts. In the present study an average of early Eocene). 200 nonmicaceous grains were counted in each sample. Sandstone, conglomerate matrix, and Techniques limestone samples were examined in thin sec- Mapping, sampling, section measuring, and tion. Where possible, 200 to 250 counts on light collection of detailed information concerning minerals were made on each sandstone and sedimentary structures were conducted for a conglomerate matrix thin section. These petro- total of seven months. One hundred seventy- graphic data are on file in the Department of one sandstone samples were examined in hand Geology, The University of Michigan, Ann specimens. The heavy minerals from 131 of Arbor, Michigan. these samples were studied, and grain counts A maximum grain-size parameter was ob- were made on 142 sandstone thin sections. tained from 70 of the sandstone samples arbi- Both heavy minerals and thin sections were trarily selected to cover the geographic distri- studied in 102 of the samples. Thin sections of bution of the Pass Peak Formation and associ- other rock types were also examined and de- ated arkose as evenly as sample coverage would scribed. allow. The observed maximum diameter of the The distribution of sampling over the area ten largest grains was measured and averaged. was not randomized because of the irregular The result was used as an index of maximum distribution of exposures. Instead, localities grain size for respective samples. were chosen to cover the geographic and strati- A grid of 1-sq mi units was superposed on graphic range of the Pass Peak Formation and the base map showing the locations of current associated arkosic deposits as completely as direction data. Thus each paleocurrent direc- time and accessibility would allow. Samples tion was located by its coordinates. A computer used for comparative purposes were collected program, utilizing an automatic plotter, was from the Pinyon Conglomerate (Paleocene), used to correct data for tectonic tilt where the Camp Davis Formation (Pliocene), and the necessary, to calculate statistical parameters, LaBarge Member and the New Fork Tongue and to plot a map of the smoothed data. of the (Eocene). Trends in the area distribution of composi- Approximately 950 paleocurrent directions tion and grain size were determined using com- were determined from cross-stratification. Ex- puter trend-surface mapping. This technique posure was rarely sufficient to permit determi- fits first-, second-, and third-degree surfaces to nation of the orientation of axes of trough the data using the least squares method. The cross-bedding and many directions of maximum surface which best fits the data is indicated by dip were measured on trough flanks. Measur- the amount of reduction of the sum of squares. able pebble imbrication was observed, and Howarth (1967) described the basis for using twelve readings were taken at three locations. the percent sum of squares to test the signifi- Large accumulations of plant debris were seen cance of a trend. on bedding planes, but no preferred orienta- The samples used for both compositional and tion could be determined. size trends were selected by superposing a grid Pebble and cobble-size and composition of 1-sq mi units on the sample location map and counts were made in the field. Samples were randomly choosing one sample from those taken at 6-in. or 1-ft intervals depending on available in each unit segment. Stratigraphic the coarseness of the conglomerate. Each clast separation of samples is small over much of the

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area studied, particularly that part south and and in the northern Green River Basin. Con- east of The Rim where there is little relief. glomerates and sandstones of Pliocene age crop However, in the Hoback Basin north and west out north of the Hoback Basin between the of The Rim, stratigraphic separation of the Hoback and Gros Ventre Ranges. samples may be as great as 1900 ft. Separate trend maps were computed for the Hoback Stratigraphy and Structure Basin and for the area south and east of The The distribution of the Pass Peak Formation, Rim and made it possible to assess the effect of its constituent lithofacies and their relation to stratigraphic changes in composition and grain the arkose are shown in Figure 2. In the north, size on the trend surfaces for the entire area. where the exposed thickness of the Pass Peak Formation may be as great as 3200 ft, it dips GEOLOGIC SETTING 35° SW. along a fault contact with the de- formed Paleozoic and Mesozoic sedimentary Regional Geology rocks on the flank of the Gros Ventre Range. In The area of investigation occupies approxi- the same area, folded Paleocene Hoback sand- mately 700 sq mi in Sublette and Teton stone is overlain unconformably by Pass Peak Counties in central-western Wyoming (Fig. 1). conglomerate and sandstone. This angular un- The northern one-fourth of the area is in the conformity disappears to the southwest, toward Hoback Basin; the remainder extends south- the center of the Hoback Basin, where the con- ward into the Green River Basin. The two tact is gradational through an intercalated se- basins are separated by a low drainage divide quence. Here, the exposed thickness is approxi- called The Rim, but are structurally continu- mately 1500 ft and the dip is 4° to 6° E. Sand- ous. The Hoback Basin is bounded on the west stones of the Pass Peak Formation extend by the Hoback Range and on the north and southward to Daniel, Wyoming, where they northeast by the Gros Ventre Range. The Wind River Range lies to the east of the Gros Ventre Range and extends southward forming the eastern boundary of the Green River Basin. The western edge of the basin is bounded by the Wyoming and Hoback Ranges. Rocks exposed in the bordering ranges repre- sent all periods, from through , with the possible exception of the (Wanless et d., 1955, p. 1). Thick Late Cretaceous and Tertiary deposits are exposed in the Hoback and Green River Basins and in the Mt. Leidy Highlands north of the Gros Ventre Range. Precambrian igneous and, to a lesser extent, metasedimentary rocks are exposed along the southwest front of the Gros Ventre Range and throughout the entire core of the Wind River Range. These exposures are the result of uplift along reverse faults in the relatively thin sedi- ments of the foreland. Paleozoic and Mesozoic sediments of the foreland are exposed along both north and south flanks of the Gros Ventre Range and east flanks of the Wind River Range. Miogeosynclinal sediments attain an aggregate thickness of 18,000 ft (Wanless et d., 1955, p. 1) in the Hoback and Wyoming Ranges where they are cut by a complex series of eastwardly directed thrust faults. Paleocene and Eocene rocks are exposed in the Hoback Basin, where Figure 2. Distribution of the Pass Peak Formation, they are approximately 17,000 ft thick, along its constituent lithofacies, and associated early Eocene the northern flank of the Gros Ventre Range arkose.

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intertongue with red and light-gray varie- (Oriel, 1962, p. 2170). Additional complica- gated mudstones of the Wasatch Formation. tions arise from the fact that the LaBarge The quartzite conglomerate-sandstone facies Member grades westward from these sites is composed almost entirely of conglomerate in into coarse conglomerate which can be traced the northern part of the area. Toward the northward where it has yielded early or middle south and southwest, sandstone units inter- Graybullian fossil mammals along the western- bedded with the conglomerate become increas- most part of The Rim (Dorr and Steidtmann, ingly abundant, thicker, and more widespread, in press). More faunal evidence is needed to and only channel conglomerates persist. The establish age relations consistent with the major sandstone units of the quartzite con- physical relations. glomerate-sandstone facies continue to the This conglomerate, exposed on the west end south where they become interbedded with of The Rim, has been mapped as Pass Peak by thick siltstone, silty shale, and thin limestone Foster (1943), Eardley et d., (1944) and lenses. This sequence constitutes the sandstone- Froidevaux (1968). However, it has neither siltstone facies which extends to the southern the gross lithologic character nor the heavy limit of the formation. Arkosic sandstone and mineral composition of the Pass Peak Forma- conglomerate intertongue with the eastern part tion and nowhere can it be seen to grade into of the Pass Peak Formation. Massive beds of the Pass Peak. These facts, in addition to the arkosic sandstone and conglomerate are ex- Graybullian age suggest that this conglomerate posed along the Green River just east of High- should not be considered Pass Peak Formation. way 187-189. These beds thin rapidly west- It is proposed that the name Pass Peak be ward, but persist as thin lenses of arkosic lag restricted to those rocks included in the quartz- sediment at the base of small scours in the ite conglomerate-sandstone and sandstone-silt- sandstone-siltstone facies. stone facies of the Pass Peak Formation as The regional stratigraphic relations of the described in this report. The eastern boundary Pass Peak Formation have been discussed by of the Pass Peak Formation is defined by the Steidtmann (1969a) and Dorr (1969). appearance of distinct lenses or beds of arkose. Beds of Pass Peak sandstone extend as far south Meaning of the Name as Daniel, Wyoming, and tongues of the same "Pass Peak" Formation sandstone may extend much farther. For map- Criteria for the recognition of the Pass ping purposes, however, the southern limit Peak Formation were discussed by Steidtmann should be drawn where the gray siltstones of (1969a). At that time, the arkose to the east the Pass Peak Formation give way to the red was considered a facies of the Pass Peak Forma- and gray variegated siltstones of the LaBarge tion. Recent information, however, has pointed Member of the Wasatch Formation. On the out that this arkose is part of a belt of arkosic west, the yellow sandstone of the Pass Peak sediment which parallels the Wind River Formation can be distinguished from the gray, Range for almost its entire length (Oriel, 1970, "salt and pepper" sandstone of the underlying oral commun.) and which may be as thick as Hoback Formation and on the north, Pass Peak 8000 ft (Shaughnessy, 1970, oral commun.). In conglomerate and sandstone can easily be dif- light of this information and because of the ferentiated from the older rocks in the Gros distinctive lithology, the inclusion of only the Ventre Range. northern part of the arkose in the Pass Peak Formation no longer seems justified. PETROGRAPHY The age of the Pass Peak Formation ranges from late Graybullian through Lysitean and Quartzite Pebble Conglomerate possibly into early Lostcabinian (Dorr, 1969). The Pass Peak conglomerate consists of ex- Sandstones of the Pass Peak Formation extend tremely well-rounded pebbles, cobbles, and southward to Daniel, Wyoming, where they boulders in a sandstone matrix (Fig. 3 A). intertongue with red and gray variegated mud- Almost all of the clasts are pressure marked at stones of the LaBarge Member of the Wasatch each contact point. Around each mark there is Formation as identified by Oriel (1967, oral a rim of calcite-cemented sandstone. Fractures commun.). However, the age of the LaBarge commonly radiate from each mark so that even Member, determined from collected be- apparently uncrushed fragments are easily tween 12 and 40 mi south of Daniel, is con- broken. Many clasts are highly polished (Dorr, sidered to be Lysitean(P) and Lostcabinian 1966). Counts indicate that 90 to 100 percent

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mum size examined was 40 cm. The size mea- surements do not include the large blocks of sandstone. All the quartzite clasts are extremely well-rounded, and many are highly spherical. The conglomerate matrix is similar in composi- tion to the sandstone. It is, however, coarser grained and well cemented with calcite. Sandstone Three types of sandstone were recognized. The most common is tan to yellow, friable, micaceous sandstone containing plant frag- ments, round clay blebs, and pyrite-limonite nodules. The grain size ranges from fine to coarse. Most grains are angular. The matrix is composed of clay, sericite, and very fine quartz and probably acts as cement to some extent. Limonite cement is most common, but calcite is also present in amounts that appear to be directly related to the abundance of fragmental carbonate grains. In outcrop, this sandstone includes zones of crumbled calcite cemented sandstone with septarian nodules, numerous plant fragments with associated heavy limonite stain and limey siltstone nodules containing well-preserved fossil leaves. Figure 3A. Pass Peak conglomerate consisting of Gray, calcite-cemented sandstone occurs as well-rounded pressure marked quartzite cobbles and individual beds and as irregular patches and boulders. elongate zones within the yellow sandstone, of the clasts are composed of quartzite, which is causing irregular weathered surfaces. In hand fine to medium grained, and red, tan, gray, specimens it appears to be similar to the yellow green, black, or white. The remainder of the sandstone, except for the cementation and lack clasts are granite, granite gneiss, red and black of clay blebs, pyrite nodules, and clay matrix. volcanic porphyry; and rarely sandstone, lime- Massive yellow and gray arkosic conglom- stone, and chert. There are large, angular erate and pebbly, feldspathic sandstone occur blocks of pressure—marked sandstone in outcrops to the east of Highway 187-189 and in the conglomerate just north of Pass Peak in lenticular zones at the base of the yellow (Fig. 3B). and gray sandstone units of the sandstone silt- The average maximum diameter of clasts stone facies of the Pass Peak. The average ------grain size is approximately 4 mm, but some grains are as large as 2 cm. The larger grains are composed of feldspar, quartz, quartzite, igneous rock fragments, and limestone frag- ments, and are surrounded by fine sandstone matrix. Limonite or calcite cement, or both, is present in small amounts. Lenses of limey clay-pebble conglomerate are present in each of the three types of sandstone. The average grain size is approximately 2 cm, but some grains are as large as 5 cm. The pebbles are subround and consist of light-gray limey silt and clay. The matrix is dark red- brown silty sand, well cemented with calcite. Figure 3B. Angular, pressure marked block of The grain types counted in sandstone thin Triassic sandstone in Pass Peak conglomerate, 0.25 mi sections include carbonate fragments, quartz, north of Pass Peak. chert, quartzite and polycrystalline quartz,

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Table I. SUMMARY OF SANDSTONE LIGHT MINERAL COMPOSITION

Pass Peak Formation (96 samples) Arkose (17 samples) percent percent Grain Composition Mean Max. Min. Mean Max. Min.

Quartz 58.4 79.4 39.1 46.7 67.5 3.7 Carbonate fragments 1.9 7.6 0.0 0.8 2.4 0.0 Chert 3.7 11.6 0.4 3.0 8.4 0.0 Quartzite 3.0 10.0 0.4 2.5 13.0 0.0 Rock fragments 13.2 25.5 1.4 11.4 18.0 5.6 Feldspar 17.1 44.4 6.8 33.2 72.2 11.0 Mica 2.9 8.2 0.0 2.1 7.0 0.0

noncarbonate rock fragments, feldspar, and samples. All but a few of the sandstone samples mica. Carbonate fragments are composed of are feldspathic arenite, arkosic arenite, or lithic partially to completely recrystallized micrite arenite (Gilbert, 1958, p. 293). and commonly contain unidentifiable shell The nonmagnetic, nonmicaceous heavy min- fragments and oolites. Quartz is the most abun- eral fractions from sandstones are characterized dant grain constituent in almost all of the by a relatively large amount of garnet and samples. Most grains are angular, but very opaque minerals. Other minerals are com- round grains with overgrowths and broken monly present in minor amounts. Table 2 overgrowths are not uncommon. Much of the summarizes the heavy mineral composition of quartz is strained. Both chert and quartzite 99 sandstone samples. grains are present in almost all the samples, but in relatively small amounts. Noncarbonate rock Shale and Siltstone fragments are common in most of the samples. Dark-gray to black shale, gray-green shale, The most abundant types are sandstone, sili- and gray and red siltstone are present in the ceous carbonate, phosphorite, quartz-mica Pass Peak Formation. Pure clay shale is rela- schist, and igneous rock. Large, angular feldspar tively rare, although Love (1969, oral com- grains, including microcline, orthoclase, and mun.) observed a large exposure of "puffy," some plagioclase are abundant. Biotite com- red, purple, and gray claystone to the southeast posed a small percentage of the grains in most of Pass Peak in the quartzite conglomerate- samples and some well-rounded grains of glau- sandstone facies. Clayey siltstone or silty shale conite were observed. Table 1 summarizes the is the most abundant fine clastic, commonly light mineral composition of 113 sandstone gray, gray-green, and rarely red. Bedding is Table 2. SCJMMARY OF SANDSTONE HEAVY MINERAL COMPOSITION

Pass Peak Formation (81 samples) Arkose (18 samples) percent percent Mean Max. Min. Mean Max. Mm.

Heavy mineral weight 1.4 7.6 0.1 1.6 4.2 0.1 percent of sample Magnetite weight percent 6.2 36.9 0.0 10.0 50.0 0.0 of heavy minerals Number percent of heavy minerals counted: Garnet 55.5 89.2 27.7 42.5 63.8 0.5 Opaque 23.1 36.5 4.8 19.8 64.7 8.8 Epidote 7.1 19.4 0.6 12.9 52.5 2.0 Zircon 2.6 10.5 0.0 2.6 12.2 0.0 Tourmaline 1.1 4.0 0.0 0.6 1.6 0.0 Sphene 1.3 5.8 0.0 1.0 3.2 0.0 Apatite 1.2 5.9 0.0 0.9 2.9 0.0 Hornblende 5.6 7.0 0.0 8.6 26.5 0.0 Rutile 0.4 1.9 0.0 0.6 1.4 0.0 Collophane 0.0 1.1 0.0 0.0 0.0 0.0 Alterite 5.6 25.8 0.0 9.1 14.0 4.0 Other 1 .2 7.7 0.0 1.2 5.4 0.0

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well developed in most exposures. Gastropods dimensional network which surrounds the sand- are abundant at some places in the gray, clayey stone units, except where truncated by chan- siltstone, and some vertebrate remains have neling. been found. Shale and siltstone samples were Limestone bodies occur just above major not examined in thin section. sandstone units and are interbedded with gray siltstone and shale. Where they overlie sand- Limestone stone units, they appear to have approximately Three types of limestone were recognized. the same shape in plan view. They are, how- The most common is brown, thin bedded to ever, much thinner; the maximum thickness massive, dense, and very brittle. Whole and observed is 10 ft. fragmental gastropods are abundant locally together with ostracod and gastropod frag- Channeling ments and possibly some oogonia. Gray and Large conglomerate-filled channels in sand- red-brown mottled limestone is also abundant. stone are well exposed along Highway 187-189 It is very thin bedded and contains algal struc- near The Rim (Fig. 4A). Both the upper and tures which cause well-developed parting. lower contacts of almost all of these channel Black, massive, very dense limestone containing conglomerates are sharp and well defined with recrystallized fossil fragments is present at a the lower contact somewhat more irregular few locations. than the upper. The only exception is where longitudinal sections show cross-bedding in the SEDIMENTARY STRUCTURES conglomerate which grades and interfingers downward into coarse sandstone. Channel con- Geometry of Lithosomes glomerates vary in thickness from a few feet to The geometry of the lithosomes is imper- 20 ft. In cross section most display a pro- fectly known because of poor exposure over nounced lenticular shape and in some cases thin much of the area. The shapes and sizes of these to only a few inches at the edges. The lower bodies can only be estimated by interpolation contacts are concave upward. The upper con- between outcrops. In the north, the conglom- tacts are convex upward, probably as a result of erate body is a prism (Krynine, 1948, Fig. 9) differential compaction. Conglomerate-filled consisting of individual lenticular sheets. It ap- channels were not observed in siltstone or shale. pears to thin to the south and west, at first be- A well-defined gradation of pebble and coming more tabular and finally separating into cobble size is not evident within the conglom- shoestring-like bodies only a few feet thick and erate channels. Rather, the conglomerate unit up to 10 ft wide. is composed of small pods or lenses containing The major sandstone bodies exposed in the pebbles of similar size, irregularly interbedded Hoback Basin are tabular. The largest of these with lenses containing pebbles of a different are approximately 80 ft thick, but 40 to 60 ft is size. Locally, the internal structure of the most common. Some are only 10 to 20 ft thick. The lateral extent of these bodies is directly proportional to their thickness; the maximum thickness is 2 to 3 mi for the thickest units and as little as 200 ft for the thinnest unit. Whether the true maximum lateral extent was observed is not known. The thickness of any of these units is essentially constant, except where they thin and pinch out very rapidly. Sandstone units in the southern part of the area appear to be thinner and of greater lateral extent than those in the Hoback Basin. The siltstone and shale body is an envelope enclosing the sandstone units. Individual beds, separated by sandstone units, are from a few Figure 4A. Cross-bedded conglomerate exposed in feet to 80 ft thick and can be traced laterally longitudinal section of a channel at top of road cut. for at least 3 mi. Whereas the sandstone bodies Lower conglomerate shows the lenticular shape of a are laterally discontinuous, the siltstone and filled channel and indistinct cross-bedding brought out shale beds are interconnected to form a three- by irregular cementation parallel to cross-strata.

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Figure 4B. Sandstone-filled channel in siltstone showing lenticular shape and steplike scours at base. channel conglomerates consists of large-scale, ate-filled channels is present along Highway but indistinct, cross-bedding. 187-189 just west of The Rim (Fig. 4A). It was Large sandstone-filled channels in siltstone not observed in the massive conglomerates in and shale are present throughout the area. An the northern part of the area. The cross-strati- outstanding example, similar to those seen else- fication is generally quite indistinct because the where is exposed in the roadcut along Highway high sphericity of the clasts precludes preferred 187-189 at The Rim (Fig. 4B). Most channel orientation on bedding surfaces. As a result, the sandstones display upper and lower bounding conglomerate appears to be massive, except surfaces which are sharp and well defined. The where interfingering sandstone or irregular ce- lower contacts are concave upward and some- mentation parallel to the bedding emphasizes what irregular because of steplike scours. The the internal structure. upper surfaces are generally very smooth and One cross-bed set occupies an entire channel convex upward as the result of differential so that the lower and upper bounding surfaces compaction. Channel-fill thickness ranges from of the set are the bottom and top of the channel. a few feet to 25 ft. Even the largest of these The sets are up to 20 ft thick. The infilling thins to a feather edge. cross-beds are straight near the top and curved Very coarse pebbly sand occupies the lower near the bottom, becoming almost tangential part of most of the channels. This grades up- with the lower bounding surface. The average ward and outward into medium and, in some dip of these cross-beds is 14°. The exact geom- places, fine sand. The bedding at the base con- etry of this cross-bedding is not known because forms to the shape of the channel, but within a it was observed only in longitudinal and oblique few feet of the bottom it becomes irregularly sections. cross-laminated because of local scour and fill. Five types of cross-stratification were ob- Some of these scour and fill structures within served in sandstone. Large-scale trough cross- the sandstone are of such magnitude that they stratification is present in sets from 1 to 3 ft may be considered filled channels within the thick. The lower boundary is erosional, concave sandstone. As in the major channels, the bed- upward, and plunges in one direction. The in- ding at the base of these large scours conforms filling laminae are most commonly discordant to the base of the scour and grades upward into with the lower bounding surface. They are irregular cross-bedding. straight near the top and curved near the Many small channels within the sandstone bottom, becoming almost tangential. Each set are filled with limey, clay-pebble conglomerate. is truncated by the set above. This structure is In general these are not more than 4 ft thick found throughout the entire area. In the quartz- and 20 ft in lateral extent. Most are much ite conglomerate-sandstone facies of the Pass smaller and probably represent only very local Peak and the arkosic sediments to the east, it is scour and fill. characterized by extreme inference between the sets and a high degree of discordance between Cross-Stratification the infilling laminae and the lower bounding Large-scale cross-stratification in conglomer- surfaces. In the sandstone units of the sand-

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RESULTS OF DATA ANALYSIS Pebble and Cobble Size The average maximum diameter of the largest pebbles or cobbles at 16 locations is shown in Figure 6. The data are unevenly distributed, but generally cover the area over which the conglomerate is exposed. A decrease in pebble and cobble size to the south is ap- parent, but the few data and their variability preclude meaningful contouring or trend-sur- face mapping. Grain Size The average size (in phi units) of the 10 largest quartz grains measured in thin section from each of 70 samples distributed over the entire area is expressed by the second-degree trend surface in Figure 6. The percent reduc- tion in the sum of squares for this surface is 16.6. The critical value for a second-degree sur- face is 12.0 at the 0.05 level of significance (Howarth, 1967, p. 624).

Figure 7. Orientation of vector means of current direction data from the Pass Peak Formation and equivalent arkose. Averaged in 1-mi grid segments. The surface in Figure 6 shows a rapid de- crease in grain size from the northeast to the center of the area of outcrop of the Pass Peak Formation and a further slight decrease to the northwest and southeast. The similarity of this surface to that computed on data exclusive of that from the Hoback Basin indicates that the trends in grain size over the entire area are not significantly affected by stratigraphic variation in the data representing the large stratigraphic interval exposed in the Hoback Basin. Current Direction Vector means from 955 rotated cross-bedding readings, analyzed in grid segments 1 mi on a side are shown in Figure 7. Each mean was tested for significance at the 0.05 level assuming both unimodal and bimodal (at 180°) distribu- tions. The Rayleigh test for uniformity of two- dimensional orientation data modified for small sample size (Durand and Greenwood, 1958, p. 234) was used. Figure 8 shows the results of smoothing the vector means in 1-mi grid seg- Figure 6. Distribution of average often largest clasts ments. The current rose and grand mean in in quartzite conglomerate and second-degree trend sur- each of these figures is a summary of all data. face on average of ten largest quartz grains in sandstone. The vector means in Figure 7 show a general

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that, therefore, they may reflect separate dis- persal systems; (2) all three constituents are present in the samples analyzed and in rela- tively large amounts in most; and (3) they may be diagnostic in terms of source-rock composi- tion. Although other grain types may be diag- nostic in terms of composition, most are not present in all of the samples, and their fre- quencies are generally low. The percent frequency of garnet in the heavy mineral fractions of 67 samples distributed over the entire area is expressed by the cubic trend surface in Figure 9. The percent reduction in the sum of squares for this surface is 54.5. The critical value for a cubic surface is 16.2 at the 0.05 level of significance (Howarth, 1967, p. 624). Over most of the area, the garnet fre- quency is between 40 and 60 percent with a slight tendency toward a decrease from north- west to southeast indicated by the shape of the 50-percent isopleth. A rapid decrease in garnet frequency from west to east is shown by iso- pleths in both the eastern and western portions of the area. This surface is similar to that com- puted on data exclusive of the Hoback Basin. Figure 8. Orientation of vector means in 1-mi grid segments (that is, data in Fig. 7) smoothed over the entire area. transport direction from north to south as do the current rose and grand mean. Current lineation and pebble imbrication generally con- form to the adjacent cross-bedding dip direc- tions. A high degree of variability is evident, par- ticularly in the northwest. Six of the vector means are not significant at the 0.05 level when both unimodal and bimodal distribution are tested and 16 means are significant only when a bimodal distribution is assumed. Figure 8 shows the vector means in unit grid segments (that is, the data in Fig. 7) smoothed over the entire area. Variability in the northwest is still ap- parent, and the general current direction to the south is emphasized by the conforming nature of the smoothed vectors. Light and Heavy Mineral Composition Trends in the areal distribution of garnet, rock fragments, and feldspar percent frequen- cies were determined using trend-surface map- EXPLANATION

ping. These particular minerals were selected 60 Garnet ilo(jlelh (peKertt) • Gridded doW I from the observed grain categories for the fol- lowing reasons: (1) cursory examination of the Figure 9. Third-degree trend surface of the percent data suggested that garnet and feldspar are garnet in the heavy minerals from Pass Peak sandstone inversely related in the samples analyzed and and equivalent arkose.

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This similarity suggests that the trends in garnet frequency over the entire area are not significantly affected by stratigraphic variation in the data representing the large stratigraphic interval exposed in the Hoback Basin. The percent frequency of feldspar in the light minerals from 67 samples distributed over the entire area is expressed by the cubic trend surface in Figure 10. The percent reduction in the sum of squares is 35.2 and is significant at the 0.05 level. The trend shown by this surface indicates a decrease in feldspar frequency from the northeast to the south-central portion of the area and a further decrease to the north- west and southeast. As before, the similarity of this surface to that computed on data exclusive of the Hoback Basin indicates that the trend in feldspar frequency over the entire area is not significantly affected by stratigraphic variation in the data from the Hoback Basin. No significant trends in the frequency of rock fragments were observed.

INTERPRETATIONS

Sediment Dispersal and Paleocurrent System EXPLANATION The general decrease in pebble and cobble 25 Feldspar .sopletK (percent} • Gridded data lac size toward the south (Fig. 6) indicates that coarse detritus was deposited by a current sys- Figure 10. Third-degree trend surface of the per- cent feldspar in Pass Peak sandstone and equivalent tem from the north. The decrease in sand size arkose. from the northeast to the center of the area and then southward (Fig. 6) indicates a separate direction also is suggested by the large blocks of current system which flowed into the basin Triassic sandstone in the conglomerate just from the northeast and continued southward. north of Pass Peak. These could not have The decrease in grain size to the northwest is traveled far without rounding and fracturing not consistent with the trend indicated by the and must have come from the uplifted sedimen- pebble-size data. One or both of the following tary rocks now exposed immediately to the situations may have been responsible for this north on the flank of the Gros Ventre Range. apparent inconsistency: (1) sand associated Observations on the distribution of the arkose with the pebbles and cobbles is generally finer are less definitive in terms of sediment dispersal than that in other parts of the area. Since the because of poor exposure. However, beds of grain size data represent maximum size, a trend arkosic sandstone thin westward and inter- in this finer sand would be masked by the major tongue with feldspathic and lithic sandstone trend over the area. (2) Size trends in sediments suggesting that the arkosic sediments were from the northeast would dominate the general brought into the basin by a current system from trend if these sediments constituted the great- the east. est proportion of the total sediment brought Trends in the percent frequency of garnet into the basin and if there was sufficient sedi- and feldspar (Figs. 9 and 10) also indicate that ment mixing. two separate current systems brought sediment The quartzite conglomerate-sandstone facies into the basin. The general decrease in garnet of the Pass Peak Formation and arkose to the frequency from west to east and the slight de- east apparently represent basin margin deposi- crease from northwest to southeast indicates an tion. The thinning and pinching out of the easterly or southeasterly transport direction for conglomerate to the south and southwest indi- the garnet-bearing sediment. The decrease in cates a transport direction to the south across feldspar frequency from the northeast to the the northern part of the area. This transport south-central portion of the area and a further

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decrease to the southeast and northwest sug- petrographic characteristics indicate the follow- gests a current system which flowed from the ing: (1) a source to the northeast with high northeast into the central portion of the basin relief in primarily igneous or metamorphic ter- and continued to the south. The further de- rane, or both, capable of supplying a great crease in feldspar to the northwest may be the amount of arkosic sediment; and (2) a source result of dilution of the sediment from the east to the north in terrane capable of supplying by that brought from the north. large blocks of Triassic sandstone, well-rounded The current system from the west, indicated quartzite pebbles and cobbles, and garnet- by the trend in garnet frequency, is not con- bearing sand. sistent with the other dispersal patterns postu- Sedimentary rocks of Paleozoic and Mesozoic lated. This and the fact that this writer knows age now exposed in the Gros Ventre Range of no western source for a great amount of gar- were a northern source for some detritus in- net (source areas to be discussed more fully cluding large blocks of Triassic sandstone, chert, under Provenance) suggest that this trend does cellophane, glauconite, and quartz grains with not reflect the true dispersal pattern of sedi- overgrowths. The Paleocene Pinyon Conglom- ments characterized by high garnet frequency. erate, now exposed to the north of the Gros Instead, this trend may reflect the dilution of Ventre Range in the Mt. Leidy Highlands, garnet-bearing sands from the north by a probably supplied the rounded quartzite clasts greater quantity of arkosic sand from the east, and associated sand to the quartzite conglom- a situation similar to that postulated to explain eratic-sandstone facies of the Pass Peak. Heavy the absence of an increase in sand size to the mineral analyses of Pinyon Conglomerate ma- north. trix (Lindsey, 1969, written commun.) indicate The results of the analysis of directional sedi- that it has a garnet content similar to that of mentary structures indicate a general current the Pass Peak Formation. It is, therefore, likely flow from north to south. This is consistent that the Pinyon Conglomerate was the source with the general pattern of transport indicated of the garnet as well as the quartzite cobbles in by the other evidence previously cited, except the Pass Peak, and the decrease in garnet fre- that separate current systems from the north quency from west to east (Fig. 9) is the result and northeast are not delineated. The failure of of sediment mixing and dilution as previously the directional data to be more definitive prob- postulated. ably stems from one or more of the following: The core of the northern Wind River Range (1) the irregular distribution of the data; (2) satisfies all the requirements for a northeast the variability inherent in the formation of di- sediment source. Its present characteristics indi- rectional structures; and (3) operator error. cate that it was of sufficient size, composition, Interpretations of the transport directions and relief to supply a great quantity of un- during the time of deposition of the Pass Peak weathered arkosic sediment with associated and equivalent beds are summarized by the minerals of igneous or metamorphic origin, or large shaded arrows in Figure 8. both. Provenance Depositional Environments Certain grain types observed in thin sections Alluvial Fan. The semicircular shape, loca- of Pass Peak and equivalent sandstones are par- tion against the Gros Ventre front, and coarse- ticularly diagnostic of provenance. Strained ness of the detritus of the quartzite conglomer- quartz, quartzite, quartz mica schist, igneous ate-sandstone facies strongly suggest deposition rock, and unweathered feldspar fragments sug- in an alluvial fan environment (Eckis, 1928; gest an igneous or metamorphic source, or both. Beaty, 1963; Bull, 1963; Denny, 1965). The Rounded quartz with broken overgrowths, large angular blocks of sandstone in the extreme chert, sandstone, cellophane, and round glau- northerly exposures of this facies probably conite indicate a sedimentary source. In addi- represent talus deposits and indicate proximity tion, the well-rounded quartzite clasts in the to source. conglomerate indicate either a distant meta- The decrease in pebble and cobble size and morphic source or a conglomerate source at an the increase in the number and thickness of unspecified distance. The blocks of Triassic sandstone units interbedded with the fanglom- sandstone indicate an adjacent sedimentary erate indicate a decrease in stream competence source. down the depositional slope, as described by Sediment dispersal patterns and the above Lustig (1963). The cross-bedded fanglomerate

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channel fill in the southern part of the fan ments and overbank deposition is reduced to a (Fig. 4A) indicates that stream flow rather minimum (Moody-Stuart, 1966, p. 1104) due than mudflow or sheetflow was the mode of to the ease with which the streams can comb transport (Blissenbach, 1954, p. 186; Bluck, across the floodplain (Allen, 1965, p. 163). After 1965, p. 232). Mudflow transport was probably the stream has aggraded for some time, its bed prohibited by the incoherent nature of the is raised above the floodplain, and it changes its debris due to the lack oi: clay associated with course suddenly by avulsion. the coarse material. Pebble imbrication in the Trough cross-stratification and discontinuous north indicates that there was some stream horizontal bedding in the arkose is similar to flow transport near the apex of the fan (Bluck, the sedimentary structures described from 1965, p. 233), but the lense-shaped units sug- Holocene deposits of braided streams by gest that sheet-flow transport was dominant. Doeglas (1962, p. 171) and Ore (1964, p. 10). The individual pods and lenses composing the Interference between sets of trough cross- fanglomerate units indicate that flow was inter- stratification indicate that they formed by the mittent, a characteristic of alluvial fan deposi- filling of spoon-shaped scours rather than from tion and a response to periods of high rainfall the migration of lunate dunes (Ore, 1964, p. in the source area. 11). Discontinuous horizontal bedding is Large-scale trough cross-bedding in the sand- formed by the superposition and interference stone units interbedded with fanglomerate is of channel-bottom deposits. similar to that reported in alluvial fan deposits Floodplain. Sediments of the sandstone- by Blissenbach (1954, p. 186), Howard (1966, siltstone facies of the Pass Peak represent a p. 148), and Nilsen (1968). Allen (1963, p. 110) typical floodplain sequence. The tabular sand- attributed this structure to the filling of spoon- stone bodies encompassed by a shale-siltstone shaped scours or to the migration of large-scale envelope, the cross-bedded, scoured, and chan- lunate dunes over the bottom. Filling of scours neled nature of the sandstone, and the presence is the most likely origin for these deposits as of both terrestrial and aquatic fossils are indica- indicated by the extreme interference between tive of floodplain deposition (Allen, 1965). adjacent sets and their association with scours Channel, overbank, and transitional deposits and channels. Large-scale, very low-dipping (Allen, 1965, p. 127) are present. The channel planar to slightly curved cross-stratification in deposits are represented by channel-lag gravel the sandstone units of the alluvial fan deposits and bar sandstone, the overbank deposits by is probably the result of the filling of channels. natural levees and floodplain siltstone, and the Allen (1963, p. 104) reported that cross-bed- transitional deposits by limestone. ding of this type forms by accretion on the The lenticular beds of arkosic gravel within gentle channelward slope of the point bars, sandstone units are lag deposits which accumu- but it is doubtful that point bars would form late as residual material during the normal proc- in the flow conditions found on alluvial fans. ess of stream action and are concentrated in Alluvial Plain. The arkose which inter- the deepest part of channels and scours. tongues with the Pass Peak Formation on the The major sandstone units consist almost east probably represents deposition on an entirely of bar deposits. Evidence as to whether alluvial plain. Although the meager exposure of these are point bar sands, as in a meandering this facies does not permit a conclusive interpre- stream, or channel bar sands, as in a low sinu- tation, the location and inferred distribution of osity or braided stream, is inconclusive. Both this facies with respect to its source in the types of deposition may be represented. Fea- Wind River Range, suggests a broad apron of tures of the sandstone units which have been sediment which paralleled the trend of the recognized in "modern" point bar deposits in- range. Its present distance from the mountain clude accumulations of plant debris (Frazier front and the absence of deep channeling and and Osanik 1961, p. 135; Harms et al., 1963, p. very coarse detritus indicate deposition some- 570), trough cross-stratification (Frazier and what farther from the source than an alluvial Osanik, 1961, p. 124; Bernard and Major, fan. 1963; Harms et al., 1963, p. 570), and planar The transport and deposition of sediments on cross-stratification (Harms et al., 1963, p. 576; the alluvial plain was accomplished by braided Potter and Pettijohn, 1963, PI. 8B). Steplike streams as indicated by the absence of fine scours (Fig. 4B), caused by lateral cutting of clastic deposits of overbank origin. Braided the stream, and large-scale, very low-dipping stream deposits are dominated by channel sedi- planar cross-stratification (Fig. 5 A) are thought

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to be diagnostic of meandering streams plant fragments, and heavy iron stain are simi- (Moody-Stuart, 1966, p. 1105), and, therefore, lar to the deposits interpreted as natural levees the sandstones containing such an association by Moody-Stuart (1966, p. 1108). The lenticu- represent point bar deposition. Following the lar shape of these zones suggest braided stream lines of reasoning of Moody-Stuart (1966, p. deposition. A thin sheet of levee sediments 1106) the association of cross-stratification overlying point bar deposits would be expected shown in Figure 5A is interpreted as originating in deposits of a meandering stream while only in a meandering stream between 50 and 70 ft poorly preserved lenticular levee deposits, as wide and having a depth of from 5 to 7 ft. A de- observed in this study, would be expected in a tailed discussion of this interpretation is pre- braided stream sequence. The limestone of the sented by Steidtmann (1970). Trough and sandstone-siltstone facies may represent transi- planar cross-stratification, however, are not tional deposition. These deposits must have limited to meandering streams (Doeglas, 1962, originated by biochemical precipitation from p. 171; Ore, 1964, p. 11), and periodic braided the standing water in small lakes which occu- conditions are not precluded. pied cut-off channels no longer receiving clastic The shale and siltstones of the sandstone- debris. siltstone facies are overbank deposits and repre- sent the settling of suspended sediment from CONCLUSIONS standing water which remained after the Arkosic detritus and sediments of the early streams flooded their banks. Sandstone chan- Eocene Pass Peak Formation were deposited in nels in the siltstone formed either when mean- the rapidly sinking northern edge of an inter- dering streams migrated across the floodplain or montane basin flanked on the north and east by when braided streams filled their channels and the rising Gros Ventre and Wind River Ranges, rapidly changed course. Channeling of the and on the west by the Hoback and Wyoming overbank deposits resulted in the reworking of Ranges. The basin was filled from the north by clayey sediments, the subsequent deposition of quartzite boulders and sand derived from the clay blebs and balls, and the formation of lenses Pinyon Conglomerate. The coarsest debris was of limey clay-pebble conglomerate in the bar deposited on an alluvial fan along the northern deposits. edge of the basin and the sand was carried The zones of crumbled, calcite-cemented southward onto a floodplain. Arkosic sediments, sandstone with associated septarian nodules, derived from the exposed Precambrian core of

Figure 11. Model of early Eocene depositions! environments and a paleogeographic reconstruction of the Hoback and northern Green River basins. Older Tertiary basin deposits not represented. Shaded area represents coarse alluvial fan deposits. Not drawn to scale.

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the rising Wind River Range, were deposited Bluck, B. J., The sedimentary history of some along the eastern edge of the basin by streams Triassic conglomerates in the Vale of Glamor- which combed across an alluvial plain in front gan, South Wales: Sedimentology, Vol. 4, p. of fans now covered by younger deposits. These 225-245, 1965. Bull, W. B., Alluvial fan deposits in western Fresno two current systems coalesced in the central County, California: J. Geol., Vol. 71, p. 243- portion of the basin and continued southward. 250, 1963. As a result, a floodplain sequence consisting of Denny, C. S., Alluvial fans in the Death Valley tabular bar deposits channeled into fine-grained region, California, Nevada: U. S. Geol. Surv., overbank sediments was deposited. Little, if Prof. Paper 466, 62 p., 1965. any, debris was shed by the ranges to the west. Doeglas, D. J., The structure of sedimentary The major elements of the model of early deposits of braided rivers: Sedimentology, Eocene deposition and a paleogeographic recon- Vol. 1, p. 167-190, 1962. struction of the Hoback and northern Green Dorr, J. A., Jr., Early Cenozoic vertebrate pale- ontology, sedimentation and orogeny in cen- River basins is shown in Figure 11. tral western Wyoming: Geol. Soc. Amer., ACKNOWLEDGMENTS Bull., Vol. 69, p. 1217-1244, 1958. Dorr, J. A., Jr., Wind-polished stones: two similar This paper is part of a dissertation submitted sites: Michigan Acad. Sci., Arts and Letters, to The University of Michigan in partial fulfill- Vol. 51, p. 265-273, 1966. ment of the requirements for the degree of Dorr, J. A., Jr., Mammalian and other fossils, Doctor of Philosophy. Professor L. I. Briggs, early Eocene Pass Peak Formation, central my committee chairman, offered helpful sug- western Wyoming: Contrib. Mus. Paleont., gestions regarding field techniques, data analy- Michigan, Univ., Vol. 22, p. 207-219, 1969. Dorr, J. A., Jr., and Steidtmann, J. R., Strati- sis, and manuscript preparation. Professor John graphic-tectonic implications of a new, earliest A. Dorr, Jr., suggested the project and guided Eocene, Mammalian Faunule, central western the field work. Dr. J. D. Love and Dr. Steven Wyoming: Michigan Acad. Sci. (in press). S. Oriel of the U.S. Geological Survey furnished Durand, D., and Greenwood, J. A., Modifications information concerning regional geology, Dar- of the Rayleigh test for uniformity in analysis win R. Spearing provided preliminary results of two dimensional orientation data: J. Geol., from his study of the Hoback Formation which Vol. 66, p. 229-238, 1958. aided in interpreting field relationships. Eardley, A. J., Horberg, L., Nelson, V. E., and Financial assistance was provided by the Church, V., Hoback-Gros Ventre-Teton field confluence map: Michigan, Univ., Con- Graduate Research Fund at The University of ference arranged by the staff of Camp Davis, Michigan, the Society of Sigma Xi, and Na- Rocky Mtn. Field Station, privately printed, tional Science Foundation Grant GA-456. 1944. Eckis, R., Alluvial fans in the Cucamonga District, southern California: J. Geol, Vol. 36, p. REFERENCES CITED 224-247, 1928. Allen, J. R. L., The classification of cross-stratified Foster, H., Structure and time relations of a por- units, with notes on their origin: Sedimentol- tion of the Hoback Range and Basin: Michi- ogy, Vol. 2, p. 93-114, 1963. gan, Univ., Ann Arbor, M. S. dissert., 47 p., Allen, J. R. L., A review of the origin and charac- 1943. teristics of recent alluvial sediments: Sedi- Frazier, D. E., and Osanik, A., Point-bar deposits, mentology (special issue), Vol. 5, p. 89-191, Old River Lockside, Louisiana: Gulf Coast 1965. Ass. Geol. Soc., Trans., Vol. 11, p. 121-137, Antweiler, J. C., and Love, J. D., Gold-bearing 1961. rocks of northwest Wyoming—a preliminary Friend, P. F., Fluviatile sedimentary structures report: U.S. Geol. Surv., Circ., Vol. 541, 12 p., in Wood Bay Series () of Spitz- 1967. bergen: Sedimentology, Vol. 5, p. 39-68, 1965. Beaty, D. B., Origin of alluvial fans, White Moun- Froidevaux, C. M., Geology of the Hoback Peak tains, California and Nevada: Ann. Asso. Amer. area in the overthrust belt, Lincoln and Sub- Geographers, Vol. 53, p. 516-535, 1963. lette counties, Wyoming: Wyoming, Univ., Bernard, H. A., and Major, C. F., Recent meander Laramie, M. S. dissert., 126 p., 1968. belt deposits of the Brazos River; an alluvial Gilbert, C. M., Sedimentary rocks in Williams, sand model; [abstr.]: Amer. Ass. Petrol. Geol., H., Turner, F. J., and Gilbert, G. M., Petrog- Bull., Vol. 47, p. 350, 1963. raphy: Freeman & Co., San Francisco, 406 p., Blissenbach, E., Geology of alluvial fans in semi- 1958. arid regions: Geol. Soc. Amer., Bull., Vol. 65, Harms, J. C., MacKenzie, D. B., and McCubbin, No. 2, p. 175-190, 1954. D. G., Stratification in modern sands of the

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